The spontaneous formation of electronic orders is a crucial element for understanding complex quantum states and engineering heterostructures in 2D materials. A novel 19$\sqrt {19} $ ×19$\sqrt {19} $ ...charge order in few‐layer‐thick 1T‐TaTe2 transition metal dichalcogenide films grown by molecular beam epitaxy, which has not been realized, is report. The photoemission and scanning probe measurements demonstrate that monolayer 1T‐TaTe2 exhibits a variety of metastable charge density wave orders, including the 19$\sqrt {19} $ × 19$\sqrt {19} $ superstructure, which can be selectively stabilized by controlling the post‐growth annealing temperature. Moreover, it is found that only the 19$\sqrt {19} $ × 19$\sqrt {19} $ order persists in 1T‐TaTe2 films thicker than a monolayer, up to 8 layers. The findings identify the previously unrealized novel electronic order in a much‐studied transition metal dichalcogenide and provide a viable route to control it within the epitaxial growth process.
A novel 19×19$\sqrt {19} \times \sqrt {19} $ charge order is observed in few‐layer thick 1T‐TaTe2 films grown by molecular beam epitaxy. The photoemission and scanning probe measurements demonstrate that monolayer 1T‐TaTe2 exhibits various metastable charge density wave orders, including the 19×19$\sqrt {19} \times \sqrt {19} $ superstructure, which can be selectively stabilized by postgrowth annealing. In 1T‐TaTe2 films thicker than a monolayer, only the 19×19$\sqrt {19} \times \sqrt {19} $ order persists.
The effect of Lifshitz transition on thermodynamics and superconductivity in hole-doped cuprates has been heavily debated but remains an open question. In particular, an observed peak of electronic ...specific heat is proposed to originate from fluctuations of a putative quantum critical point p* (e.g., the termination of pseudogap at zero temperature), which is close to but distinguishable from the Lifshitz transition in overdoped La-based cuprates where the Fermi surface transforms from hole-like to electron-like. Here we report an in situ angle-resolved photoemission spectroscopy study of three-dimensional Fermi surfaces in La2-xSrxCuO4 thin films (x = 0.06 to 0.35). With accurate kz dispersion quantification, the said Lifshitz transition is determined to happen within a finite range around x = 0.21. Normal state electronic specific heat, calculated from spectroscopy-derived band parameters, reveals a doping-dependent profile with a maximum at x = 0.21 that agrees with previous thermodynamic microcalorimetry measurements. The account of the specific heat maximum by underlying band structures excludes the need for additionally dominant contribution from the quantum fluctuations at p*. A d-wave superconducting gap smoothly across the Lifshitz transition demonstrates the insensitivity of superconductivity to the dramatic density of states enhancement.
Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the ...inter-electrode gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-electrode gap as small as 17µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000°C, the highest reported by far. We first demonstrate that electrostatic gating of graphene by a 20nm HfO2 dielectric layer changes the graphene anode work function by 0.63eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the gap from 1mm down to 17µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Finally, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-electrode gap.
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•A thermionic energy converter (TEC) with a back-gated graphene anode is demonstrated.•The efficiency increases by a factor of 30.6 by reducing the inter-electrode gap from 1 mm to 17 µm.•Electrostatic gating of graphene leads to an additional 67% enhancement to the efficiency.•The maximum electronic conversion efficiency is 9.8% at the cathode temperature of 1000 °C.
Charge density wave (CDW is one of the most ubiquitous electronic orders in quantum materials. While the essential ingredients of CDW order have been extensively studied, a comprehensive microscopic ...understanding is yet to be reached. Recent research efforts on the CDW phenomena in two-dimensional (2D) materials provide a new pathway toward a deeper understanding of its complexity. This review provides an overview of the CDW orders in 2D with atomically thin transition metal dichalcogenides (TMDCs) as the materials platform. We mainly focus on the electronic structure investigations on the epitaxially grown TMDC samples with angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy as complementary experimental tools. We discuss the possible origins of the 2D CDW, novel quantum states coexisting with them, and exotic types of charge orders that can only be realized in the 2D limit.
Quantum spin liquids are highly entangled, disordered magnetic states that are expected to arise in frustrated Mott insulators and to exhibit exotic fractional excitations such as spinons and ...chargons. Despite being electrical insulators, some quantum spin liquids are predicted to exhibit gapless itinerant spinons that yield metallic behaviour in the charge-neutral spin channel. We deposited isolated magnetic atoms onto single-layer 1T-TaSe2, a candidate gapless spin liquid, to probe how itinerant spinons couple to impurity spin centres. Using scanning tunnelling spectroscopy, we observe the emergence of new, impurity-induced resonance peaks at the 1T-TaSe2 Hubbard band edges when cobalt adatoms are positioned to have maximal spatial overlap with the local charge distribution. These resonance peaks disappear when the spatial overlap is reduced or when the magnetic impurities are replaced with nonmagnetic impurities. Theoretical simulations of a modified Anderson impurity model show that the observed peaks are consistent with a Kondo resonance induced by spinons combined with spin-charge binding effects that arise due to fluctuations of an emergent gauge field.Unconventional quasiparticles carrying spin but not electric charge emerge in quantum spin liquid phases. The Kondo interaction of these spinon quasiparticles with magnetic impurities may now have been observed.
The oxygen redox (OR) activity is conventionally considered detrimental to the stability and kinetics of batteries. However, OR reactions are often confused by irreversible oxygen oxidation. Here, ...based on high-efficiency mapping of resonant inelastic x-ray scattering of both the transition metal and oxygen, we distinguish the lattice OR in Na
Li
Mn
O
and compare it with Na
Mg
Mn
O
. Both systems display strong lattice OR activities but with distinct electrochemical stability. The comparison shows that the substantial capacity drop in Na
Li
Mn
O
stems from non-lattice oxygen oxidations, and its voltage decay from an increasing Mn redox contribution upon cycling, contrasting those in Na
Mg
Mn
O
. We conclude that lattice OR is not the ringleader of the stability issue. Instead, irreversible oxygen oxidation and the changing cationic reactions lead to the capacity and voltage fade. We argue that lattice OR and other oxygen activities should/could be studied and treated separately to achieve viable OR-based electrodes.
Many chalcogenide glasses undergo a breakdown in electronic resistance above a critical field strength. Known as threshold switching, this mechanism enables field-induced crystallization in emerging ...phase-change memory. Purely electronic as well as crystal nucleation assisted models have been employed to explain the electronic breakdown. Here, picosecond electric pulses are used to excite amorphous Ag_{4}In_{3}Sb_{67}Te_{26}. Field-dependent reversible changes in conductivity and pulse-driven crystallization are observed. The present results show that threshold switching can take place within the electric pulse on subpicosecond time scales-faster than crystals can nucleate. This supports purely electronic models of threshold switching and reveals potential applications as an ultrafast electronic switch.
A two-dimensional (2D) topological insulator exhibits the quantum spin Hall (QSH) effect, in which topologically protected conducting channels exist at the sample edges. Experimental signatures of ...the QSH effect have recently been reported in an atomically thin material, monolayer WTe
. Here, we directly image the local conductivity of monolayer WTe
using microwave impedance microscopy, establishing beyond doubt that conduction is indeed strongly localized to the physical edges at temperatures up to 77 K and above. The edge conductivity shows no gap as a function of gate voltage, and is suppressed by magnetic field as expected. We observe additional conducting features which can be explained by edge states following boundaries between topologically trivial and nontrivial regions. These observations will be critical for interpreting and improving the properties of devices incorporating WTe
. Meanwhile, they reveal the robustness of the QSH channels and the potential to engineer them in the monolayer material platform.
Taking control of spin currents Shen, Zhi-Xun; Sobota, Jonathan
Nature (London),
9/2017, Letnik:
549, Številka:
7673
Journal Article
Recenzirano
Conventional wisdom dictates that an electron's magnetic moment and momentum are strongly coupled only in materials made of heavy elements. An experiment demonstrates a striking counterexample.
Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the ...'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.
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